In recent years, the use of biofuel, which is extracted, for instance, from sugarcane, corn, or wood, is promoted. As part of such promotional efforts, studies are being conducted on mixing a light oil with a biofuel (methyl ester) made, for instance, from rapeseed oil and using the resulting mixture as a diesel engine fuel.
When using the above-mentioned mixed fuel, it is necessary, for instance, to provide fuel injection control in accordance with biofuel concentration for the purpose of maintaining, for instance, adequate ignition performance. It is therefore important that the concentration of biofuel in the fuel supplied to a diesel engine be detected with high accuracy.
An apparatus disclosed in JP-A-1993-133886 uses the permittivity difference between alcohol and gasoline, measures the capacitance between a pair of electrode plates to detect alcohol concentration, and corrects the correlations between heaviness and refractive index in accordance with the detected alcohol concentration to determine the degree of heaviness.    Patent Document 1: JP-A-1993-133886    Patent Document 2: JP-A-1998-19775    Patent Document 3: JP-A-1993-223026